FIG. 1 shows a conventional example of an OTP cell. It includes a fuse 10 connected between a programming terminal 12 and a node A. Node A is connected to high supply voltage Vdd by a current source 14. The input of an inverter 16 is connected to node A, and output S of this inverter provides a logic state corresponding to the state of fuse 10.
Further, node A is connected to low supply voltage Vss by a reset transistor MN1.
In normal operation, programming terminal 12 is connected to low supply voltage Vss. Upon circuit power-on, reset transistor MN1 receives a reset pulse INIT on its gate. Transistor MN1 is thus turned on for a short time to bring the voltage of node A to voltage Vss. After the reset pulse, transistor MN1 is off. If fuse 10 is conductive, it maintains node A to voltage Vss and inverter 16 provides logic state 1. If fuse 10 is off, current source 14 draws node A to voltage Vdd and inverter 16 then provides logic state 0.
Fuse 10 is often formed of the oxide layer separating the substrate from the gate of a MOS transistor. A P-channel MOS transistor having an interconnected drain, well and source is generally used. Thus, the unprogrammed fuse is isolating. To program the fuse, the oxide layer is broken down, to then become conductive. For this purpose, a programming voltage Vpp much greater than supply voltage Vdd is applied between the two surfaces of the oxide layer. In the cell of FIG. 1, voltage Vpp is applied on programming terminal 12 while transistor MN1 is turned on.
Programming terminal 12 is generally external to the circuit, since voltage Vpp would risk damaging the circuit components if it were transmitted internally. The short-circuit of node A to voltage Vss through transistor MN1 prevents a possible increase of the voltage on node A under the effect of programming voltage Vpp.
A disadvantage of the cell of FIG. 1 is that a current source 14 that permanently provides current when the corresponding fuse 10 is conductive is required for each fuse 10. This discourages the use of OTP cells in applications where a small consumption is desired.